In radiography the interior of objects is reproduced by means of penetrating radiation, which is high energy radiation also known as ionising radiation belonging to the class of X-rays, gamma-rays and high-energy elementary particle radiation, e.g. beta-rays, electron beam or neutron radiation. For the conversion of penetrating radiation into visible light and/or ultraviolet radiation luminescent substances, called phosphors, are used.
In a conventional radiographic system an X-ray radiograph is obtained by X-rays transmitted image-wise through an object and converted into light of corresponding intensity in a so-called intensifying screen (X-ray conversion screen) wherein phosphor particles absorb the transmitted X-rays and convert them into visible light and/or ultraviolet radiation to which a photographic film is more sensitive than to the direct impact of X-rays.
In practice the light emitted image-wise by said screen irradiates a contacting photographic silver halide emulsion layer film which after exposure is developed to form therein a silver image in conformity with the X-ray image.
More recently as described e.g. in U.S. Pat. No. 3,859,527 an X-ray recording system has been developed wherein photostimulable storage phosphors are used having in addition to their immediate light emission (prompt emission) on X-ray irradiation the property to store temporarily a large part of the X-ray energy. Said energy is set free by photostimulation in the form of fluorescent light different in wavelength from the light used in the photostimulation. In said X-ray recording system the light emitted on photostimulation is detected photoelectronically and transformed into sequential electrical signals.
The basic constituents of such X-ray imaging system operating with a photostimulable storage phosphor are an imaging sensor containing said phosphor in particulate form normally in a plate or panel, which temporarily stores the X-ray energy pattern, a scanning laser beam for photostimulation, a photoelectronic light detector providing analogue signals that are converted subsequently into digital time-series signals, normally a digital image processor which manipulates the image digitally, a signal recorder, e.g. magnetic disk or tape, and an image recorder for modulated light exposure of a photographic film or an electronic signal display unit, e.g. cathode-ray tube. A survey of lasers useful in the read-out of photostimulable latent fluorescent images is given in the periodical Research Disclosure December 1989, item 308117.
Of special interest in the application of said image recording and reproducing method are particular barium fluorohalide phosphors identified in U.S. Pat. No. 4,239,968.
According to U.S. Pat. No. 4,239,968 a method is claimed for recording and reproducing a radiation image comprising the steps of (i) causing a visible ray- or infrared ray-stimulable phosphor to absorb a radiation passing through an object, (ii) stimulating said phosphor with stimulation rays selected from visible rays and infrared rays to release the energy of the radiation stored therein as fluorescent light, characterised in that said phosphor is at least one phosphor selected from the group of alkaline earth metal fluorohalide phosphors represented by the formula: EQU (ba.sub.1-x M.sub.x.sup.II)FX:yA
wherein:
M.sup.II is one or more of Mg, Ca, Sr, Zn and Cd; PA1 X is one or more of Br, Cl or I; PA1 A is at least one member of the group consisting of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb and Er; and PA1 x is in the range 0.ltoreq.x.ltoreq.0.6 and y is in the range 0.ltoreq.y.ltoreq.0.2, and that the wavelength of said stimulating rays is not less than 500 nm. In said U.S. patent a graph shows the relationship between the wavelength of the stimulation rays and the luminance of the stimulated light, i.e. the stimulation spectrum from which can be learned that said kind of phosphor has high photostimulation sensitivity to stimulation light of a He--Ne laser beam (633 nm) but poor photostimulability below 500 nm. The stimulated light (fluorescent light) is situated in the wavelength range of 350 to 450 nm with a peak at about 390 nm (ref. the periodical Radiology, September 1983, p.834.). PA1 i. causing a radiation image storage panel containing a photostimulable phosphor to absorb radiation having passed through an object or having been radiated from an object, PA1 ii. exposing said image storage panel to stimulating rays to release the radiation energy stored therein as light emission, the stimulating rays being electromagnetic waves having a wavelength within the range of 700-900 nm PA1 iii. detecting the emitted light, characterised in that said photostimulable phosphor corresponds to the general formula: EQU Ba.sub.1-x-y"-z-r Sr.sub.x Pb.sub.y" Cs.sub.2r Eu.sub.z F.sub.2-a-b Br.sub.a I.sub.b, PA1 0.ltoreq.x.ltoreq.0.30, 10.sup.-4 &lt;y"&lt;10.sup.-3, 10.sup.-7 &lt;z&lt;0.15, 0.ltoreq.r&lt;0.05, 0.75.ltoreq.a+b.ltoreq.1.00, 0.05&lt;b&lt;0.20. PA1 M.sup.1+ is at least one alkali metal selected from the group consisting of Li, Na, K, Rb and Cs; PA1 M.sup.2+ is at least one divalent metal selected from the group consisting of Ca, Mg and Pb; PA1 M.sup.3+ is at least one trivalent metal selected from the group consisting of Al, Ga, In, Tl, Sb, Bi, Y or a trivalent lanthanide, e.g. La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; PA1 0.ltoreq.x.ltoreq.0.30, 0.ltoreq.y.ltoreq.0.10, 0.ltoreq.p.ltoreq.0.3, 0.ltoreq.q.ltoreq.0.1, 0.05.ltoreq.a.ltoreq.0.76, 0.20.ltoreq.b.ltoreq.0.90, a+b&lt;1.00 and 10.sup.-6 .ltoreq.z.ltoreq.0.2.
In radiography, as in any imaging method, the signal-to-noise ratio of the recorded image should be as high as possible.
Although BaFBr:Eu.sup.2+ storage phosphors, used in digital radiography, have a relatively high X-ray absorption in the range from 30-120 keV, which is relevant for general medical radiography, the absorption is lower than the X-ray absorption of most prompt-emitting phosphors used in screen/film radiography, like e.g. LaOBr:Tm, Gd.sub.2 O.sub.2 S:Tb and YTaO.sub.4 :Nb. Therefore, LaOBr:Tm, Gd.sub.2 O.sub.2 S:Tb or YTaO.sub.4 :Nb screens will absorb a larger fraction of the irradiated X-ray quanta than BaFBr:Eu screens of equal thickness. The signal to noise ratio (SNR) of an X-ray image being proportional to the square-root of the absorbed X-ray dose, the images made with LaOBr:Tm, Gd.sub.2 O.sub.2 S:Tb or YTaO.sub.4 :Nb screens will be less noisy than images made with BaFBr:Eu screens having the same thickness. A larger fraction of X-ray quanta will be absorbed when thicker BaFBr:Eu screens are used. Use of thicker screens, however, leads to diffusion of light over larger distances in the screen, which causes deterioration of image resolution. For this reason, X-ray images made with digital radiography, using BaFBr screens, as disclosed in U.S. Pat. No. 4,239,968, give a more noisy impression than images made with screen/film radiography.
A more appropriate way to increase the X-ray absorption of phosphor screens is by increasing the intrinsic absorption of the phosphor. In BaFBr:Eu storage phosphors this can be achieved by partly substituting bromine by iodine.
BaFX:Eu phosphors containing large amounts of iodine have been described e.g. in EP-A 142 734, the general formula of said phosphor is BaF(Br.sub.1-x I.sub.x):yEu, and 10.sup.-3 .ltoreq.x&lt;1.0. In FIG. 3 of said patent, the relative luminance of BaFX:Eu storage phosphors is shown as a function of the iodine content. It is clear from said FIG. 3 of the disclosure mentioned above, that, although x can be as great as 1.0 according to the general formula, the portion of Br that is replaced by I should preferably not be made higher than 50% % since replacement of a larger portion of Br by I, leads to a lower relative luminance, of the light emitted upon stimulation. The relative luminance of the storage phosphor should be as high as possible, since the sensitivity of a storage phosphor system is proportional to the storage phosphor luminance and apart from a high X-ray absorption, a high system sensitivity is essential for reducing image noise. Therefore, in a phosphor as disclosed in EP-A 142 734, the gain in image quality, due to the higher absorption of X-rays when more than 50% of iodine is included in the phosphor is offset by the lowering of the relative luminance.
In EP-A 533 236 a divalent europium activated stimulable phosphor is claimed wherein the stimulated light has a higher intensity when the stimulation proceeds with light of 550 nm, than when the stimulation proceeds with light of 600 nm. It is said that in said phosphor a "minor part" of bromine is replaced by chlorine and/or iodine. By minor part has to be understood less than 50 atom %.
In digital radiography it can be advantageous to use photostimulable phosphors that can very effectively be stimulated by light with wavelength higher than 600 nm, since then the choice of small reliable lasers that can be used for stimulation (e.g. He--Ne, semi-conductor lasers, solid state lasers, etc) is very great so that the laser type does not dictate the dimensions of the apparatus for reading (stimulating) the stimulable phosphor screen.
In EP-A 704 511 a radiation image recording and reproducing method is disclosed comprising the steps of:
wherein
The signal-to-noise ratio achieved in radiographic images produced by the method of EP-A 704 511 leaves however still room for improvement.
Therefore the need for stimulable phosphors, giving a better signal-to-noise ratio, a higher speed and being stimulable at wavelengths above 600 nm is still there.